The present invention relates to an exhaust gas cleaner for effectively removing nitrogen oxides from an exhaust gas containing nitrogen oxides and an excess amount of oxygen, and a method for removing nitrogen oxides with such an exhaust gas cleaner.
Various exhaust gases discharged from internal combustion engines such as automobile engines, etc., combustion apparatuses installed in factories, home fun heaters, etc. contain nitrogen oxides such as nitrogen monoxide and nitrogen dioxide together with an excess amount of oxygen. The term "containing an excess amount of oxygen" means that the oxygen content is larger than its stoichiometric amount relative to unburned components such as carbon monoxide, hydrogen, hydrocarbons in the exhaust gas. The term "nitrogen oxides" means nitrogen monoxide and/or nitrogen dioxide.
The nitrogen oxides are one of causes of acid rain, posing a serious problem of environmental pollution. For these reasons, various methods have been proposed to remove nitrogen oxides from exhaust gases emitted from various combustion equipment.
In the case of large, stationary combustion apparatuses such as large combustion apparatuses of factories, ammonia is introduced into an exhaust gas, whereby nitrogen oxides in the exhaust gas are catalytically and selectively reduced.
However, such a method is disadvantageous, because ammonia is expensive, because ammonia is so toxic that the amount of ammonia should be controlled by measuring the concentration of nitrogen oxides in the exhaust gas, and because this reduction system generally needs large apparatuses.
There have been proposed methods of reducing nitrogen oxides by adding to an exhaust gas hydrocarbons in a smaller amount than a stoichiometric amount relative to oxygen in the exhaust gas, in the presence of a catalyst such as zeolite with or without carrying a transition metal (Japanese Patent Laid-Open Nos. 63-100919, 63-283727 and 1-130735; Thesis 2A526, 1990, the 59th Spring Conference of the Japan Chemical Society; Theses 3L420, 3L422 and 3L423, 1990, the 60th Fall Conference of the Japan Chemical Society; and "Catalyst", Vol. 33, No. 2, p.59 (1991)). A catalyst carrying silver has been also proposed (Japanese Patent Laid-Open No. 4-281844).
However, these methods are effective only in a narrow temperature range of NOx removal. Also, their efficiency of removing nitrogen oxides is extremely low in the case of an actual exhaust gas because it contains about 10% of moisture.